1. Field of the Invention
The present invention relates to a transmission line, and more specifically to a transmission line configured to transmit a high frequency signal.
2. Description of the Related Art
A representative transmission line connecting a high frequency circuit, a high frequency element and the like with each other for transmitting a high frequency signal is a coaxial cable. The coaxial cable is formed of a central conductor that transmits a high frequency signal and a shield conductor surrounding the central conductor coaxially, and provides a transmission line with excellent shieldability and is also inexpensive, and accordingly, it is used for a variety of high frequency equipment. As mobile equipment is increasingly reduced in size and thickness, however, there is a demand for a cable reduced in thickness to be incorporated in the equipment.
In relation thereto, Japanese Registered Utility Model No. 3173143 proposes a triplate-type transmission line thinner than the coaxial cable and suitable for being incorporated in mobile equipment. When the thus configured transmission line has a flexible resin base material applied thereto, the transmission line can be enhanced in vertical flexibility and thus be disposed in an elongated gap along a curved surface of a casing of a mobile terminal.
In Japanese Registered Utility Model No. 3173143, however, a vertical ground conductor is connected by a via conductor, and this imposes a large burden on fabricating the transmission line.
More specifically, a ground conductor is normally produced of copper foil, conductive paste and the like, i.e., different types of materials, bonded together. When the transmission line is bent and thus used, and there is a via in a vicinity of the bent portion, the interface of the via and the copper foil experiences stress, which may cause breakage and the via may be disconnected. Furthermore, when the via is excessively close to a side surface of a base material, cutting a collective base material into individual pieces causes stress to crack the base material, and accordingly, it is necessary to provide the via inside the side surface with some margin considered. This requires the line to have a width increased more than required for a property or the like and thus imposes a large burden on fabricating the transmission line.